Comment rate

Comment rate turns raw comment counts into something comparable across videos of very different sizes. Divide comments by views and you get a percentage, usually a small one, since most people watch and say nothing at all. It is useful for spotting which topics pull an audience into conversation. The catch is that the number moves with what you asked for. A video ending on a direct question will beat one that did not, whatever the quality of either, so compare like with like or you are measuring your call to action. Controversy inflates it too, and that is a poor thing to optimise towards.